Monday, August 13, 2012

What Happened to Summer?

The recent spell of cool weather in the eastern half of the country has been a welcome respite from the brutal summer weather this year. Some may argue that most of the central and eastern U.S. has experienced two years worth of summer in two months this year.  Although August started out on the warm side, a significant change in the upper level wind patterns has brought an early taste of fall to the central U.S.

This all began at the end of the first week of August with the passage of a strong upper low through the middle of the country. As the trough developed over the eastern half of the U.S. the upper level ridge retreated westward. A second, even strong upper level low dropped into the Midwest this weekend, reinforcing the trough over the eastern U.S.

The 500 millibar map (~18,000-20,000 ft) for the morning of Friday, August 10
showing the strong upper low over the Great Lakes.

It's been a welcome turnaround. Temperatures the last week have averaged near to well below normal across much of the central U.S., while the western U.S. is experiencing the hot weather. In addition, there have been several rounds of much needed rain. It certainly hasn't broken the drought, but has slowed any worsening for the time being.

After piling up massive numbers of record high temperatures the last two months in the central U.S., record low minimum and record low maximum temperatures have been the rule the past few days. Saturday was a very cool day in Michigan with high temperatures topping out only in the low 60s, many of those records. Record low temperatures were also scattered through the upper Midwest this weekend with minimum temperature dropping into the low 40s.

Locations that set record low maximum temperatures on August 11.
A complete list of records can be viewed at the National Climatic Data Center Records Lookup page.

The surface low pressure system associated with the upper trough was of record proportions itself. The low generated gales across Lake Michigan and rare lake-effect thunderstorms dropped more than three inches of rain along the lake in northern Illinois. You can read an excellent discussion about this unusual storm system , including graphics and photos, at the National Weather Service Chicago web site.  Heavy rain also fell eastward through Michigan, with close to four inches in Bay County.

CoCoRaHS map for August 10. Note the high rainfall in northeastern Illinois and along the
south end of Saginaw Bay in Michigan.

CoCoRaHS map for August 10 showing high rainfall along the Lake Michigan shoreline in northern Illinois

So will this cool weather last? The 8-14 day outlook issued by the Climate Prediction Center on August 12th indicates a higher probability for cooler than normal weather for the central U.S.  I like that sound of that.

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